The Republicans have sent Barbara Bush into battle this week to try to win the women's vote which has consistently leaned towards the Democrats' contender for the presidency, Al Gore.

But what the offensive by the former US first lady will not address openly is the key reason that female floating voters have gone Mr Gore's way: abortion rights.

When the US vice-president announced at the Democratic convention in Los Angeles in August that he supported a woman's right to choose, he got one of the noisiest standing ovations of the week. It was an indication of the impact the abortion question can have on how women vote.

The licensing last month by the Food and Drug Administration of the RU-286 abortion pill has brought the issue to the forefront of the presidential race.

Mr Gore used the FDA decision to proclaim his support for abortion rights. Barbara Bush's son, the Republican contender George W Bush, used the occasion to say that his aim was to make abortion "more rare" in the US.

This week's Republican offensive also sees Laura Bush, the candidate's wife, and Cindy McCain, wife of his former rival for the Republican nomination, John McCain, pressed into service.

Abortion is as controversial as ever in the US where 1.3m terminations are carried out annually, but where 86% of all counties lack abortion facilities. Several doctors who carry out abortions have been murdered by pro-life activists in past years. Others have been threatened and their families harassed. Two weeks ago, a priest in Illinois drove a car into an abortion clinic and attacked the building with an axe.

When the film Cider House Rules, featuring Michael Caine as an abortion doctor, won an Oscar for its author, John Irving, for best adapted screenplay this year, Mr Irving dedicated his prize to pro-choice groups, angering the anti-abortion movement.

At the heart of the issue in terms of this election is the 1973 Roe v Wade supreme court decision which ruled that a woman's constitutional right to privacy gave her the right to choose an abortion.

Pro-abortionists fear that a supreme court packed with the conservative judges Mr Bush favours would reverse decisions on abortion and return the country to a situation where terminations were illegal. Bruce Shapiro, a journalist of the Nation magazine and a former director of a monitoring group, Supreme Court Watch, described abortion as "probably the single largest issue pulling the swing vote".

Mr Gore has now firmly declared his support for the right to choose although this has not always been his position.

In 1984, he wrote to a student saying that "it is my deep personal conviction that abortion is wrong" and in 1977 he voted to ban federal funds for abortions for poor women unless the woman's life was in danger.

His running mate, Joe Lieberman, has not always been wholehearted in support of abortion either, but both have become born-again pro-choice advocates for the election.

Mr Bush, in contrast, has said he will "do everything in my power" to restrict abortion. He is opposed to all abortion except in cases of rape, incest and danger to the health of the mother.

He also appears to have shifted his views over the years. He was reported in the Lubbock, Texas Avalanche-Journal in 1973 as saying that, while he was personally against abortion, he believed the matter should be left to the woman and her doctor. His team now denies this report.

In February Mr Bush won the backing of the National Right to Life Committee and he supports proposals for a constitutional amendment banning abortion. He has declared his affinity with the two most outspoken critics of abortion in the supreme court, Clarence Thomas and Antonin Scalia.

"Governor Bush realises this is a controversial issue," said spokeswoman Mindy Tucker yesterday. "But he wants to bring people together on what we can agree on: reducing abortions in America."

The major pro- and anti-organisations have both put big money into the race. The National Abortion and Reproductive Rights League (Naral) said yesterday it would be spending $5m (£3m) to help Mr Gore.